In a wireless communications system, if a transmit end (for example, a base station) can learn of channel state information (channel status information, CSI for short) in some manner, the transmit end can optimize a to-be-sent signal based on channel characteristics, to improve signal received quality and overall system performance.
However, in an FDD system, CSI can usually be accurately obtained only by a receive end. When a transmit end needs to obtain CSI, the receive end needs to feed back information related to the CSI to the transmit end. For example, in a Long Term Evolution (Long Term Evolution, LTE for short) system, user equipment (user equipment, UE for short) does not directly feed back channel information, but recommends a precoding matrix to a transmit end such as a base station. The precoding matrix is used by the base station to precode data. For example, the UE may feed back a precoding matrix indicator (precoding matrix indicator, PMI for short) to the base station, and the PMI may be based on a double codebook structure. Based on the double codebook structure, the PMI may be split into a PMI 1 and a PMI 2. The PMI 1 is used to select a group of basis vectors, and the PMI 2 is used to select at least one basis vector from the group of basis vectors selected by using the PMI 1, and select a phase difference between two polarization directions. After receiving the PMI 1 and the PMI 2, the base station finds corresponding code words W1 and W2 in a stored codebook C and obtains a precoding matrix according to a preset function rule F(W1, W2). In addition, the user equipment has various codebook feedback manners.
PMI feedback is related to a data transmission method used by the base station. In a transmit diversity (for example, space frequency block coding, spatial frequency block coding, SFBC for short) transmission method, the base station sends a demodulation reference signal (DM-RS) and data to the user equipment through two antenna ports. To determine two antenna ports through which data is sent to the user equipment, a receive end needs to estimate state information of a channel from the base station to the receive end and feed back a PMI to the base station.
In a next generation wireless communications system, to resist severe attenuation of high-band signals, more antennas are required. Consequently, beams become narrower and are more easily blocked. In the prior art, beams corresponding to two ports in SFBC are basically consistent in direction, and therefore, when beam blocking occurs, the beams corresponding to the two ports are blocked, which severely affects transmission performance. In addition, data or a DM-RS on each port is precoded by using a plurality of antennas in only one polarization direction, and a beamforming gain generated when all antennas in two polarization directions are used for precoding cannot be obtained.